1. Field of the Invention
The present invention relates to a switching power supply unit and an electronic apparatus including such a switching power supply unit.
2. Description of the Related Art
There has been an increasing need for reducing power consumption when, for example, printers or facsimiles are in a standby mode, or when such devices are not performing a printing operation. Among various types of needs, a reduction in power consumption is required when power supply units used for printers or facsimiles are in a standby mode, or in a light-load mode.
However, general ringing choke converter (RCC) type switching power supply units have characteristics such that, as a load is reduced, the switching frequency is increased and the switching loss is thus increased. Thus, it is difficult to reduce the power consumption in the light-load mode.
An RCC type switching power supply unit for reducing power consumption in a light-load mode is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 7-67335. The switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 7-67335 includes a circuit in which a control terminal of a first switching element is forcibly grounded for a predetermined time in the light-load mode. Thus, turning on of the first switching element is delayed, so that the switching frequency is maintained below a predetermined level. Accordingly, power consumption in the light-load mode is reduced.
However, the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 7-67335 has a problem. Since the switching frequency is merely maintained below the predetermined level, the switching frequency cannot be drastically reduced in the light-load mode. Thus, the power consumption cannot be drastically reduced.
Also, there is another problem in that the switching frequency cannot follow an abrupt load change. For example, if the switching frequency in the light-load mode is set to be significantly different from that in the heavy-load mode, the switching frequency cannot follow an abrupt change from a lighter load to a heavier load. Thus, a reduction in output or suspension of a power supply may be caused. Accordingly, there is a problem in that the switching frequency cannot be drastically reduced in the light-load mode.